This invention relates in general to electrographic reproduction apparatus, and more particularly to an electrographic reproduction apparatus having a pair of similar doner rollers for receiving and transferring images to opposite sides of a receiver to produce duplex copies.
Electrographic reproduction apparatus typically produce copies of original information on receiver members, such as cut sheets of plain bond paper for example. Such information copies may be formed on one side of the paper sheets (referred to as simplex copies), or on both sides of such sheets (referred to as duplex copies). Commercial electrographic reproduction apparatus capable of producing duplex copies are generally classified as "two-pass" or "single-pass" apparatus.
In "two-pass" reproduction apparatus, information is electrographically reproduced sequentially on the first sides of sheets which are thereafter collected in an intermediate tray. Such sheets are then sequentially transported from the intermediate tray back through the apparatus to have information respectively reproduced electrographically on the second sides of such sheets. As a result, a relatively long travel path is required for transporting sheets through the reproduction apparatus twice to produce the duplex copies. Therefore, the potential for jams or other sheet handling complications is increased. Moreover, the first completed duplex copy is not available for inspection until after all first side copies are produced. Thus considerable time elapses until the first complete duplex copy is produced, and any errors in such duplex copy are not determined until after all first side copies have already been made.
In "single-pass" reproduction apparatus selected information is electrographically produced respectively on both sides of a sheet during a single pass through such apparatus. While single-pass apparatus are successful in overcoming the noted disadvantages of two-pass electrographic reproduction apparatus, they tend to introduce, in and of themselves, other disadvantages or complications. For example, U.S. Pat. No. 3,775,102 (issued Nov. 27, 1973, in the name of Punnett) shows two separate substantially complete electrographic process assemblies for reproducing information respectively on each side of a sheet. Such duplicative assemblies require precise optical alignment, substantially increase apparatus cost, and add significant complexity (reduced reliability) to the apparatus.
To eliminate duplicative process assemblies, the single-pass reproduction apparatus may employ an electrically biased doner/transfer roller for simultaneously transferring images, corresponding to information to be reproduced, respectively to each side of a sheet, such as shown for example in U.S. Pat. No. 3,847,478 (issued November 12, 1974, in the name of Young). However, the doner/transfer roller is of complex construction and requires accurate bias control to accomplish the simultaneous image transfers. Simplification of the transfer apparatus is accomplished by utilizing spaced conventional transfer apparatus associated with a single electrographic process assembly, such as shown for example in U.S. Pat. No. 4,194,829 (issued Mar. 25, 1980, in the name of Cavagnaro). Such apparatus requires a complex, precisely controllable sheet handling mechanism which turns the sheet over between transfers to reproduce information on each side thereof.